The present invention relates to frequency modulation. In particular, the present invention relates to frequency modulation using materials that carry a sliding charge density wave.
A. Modulation
The fundamental purpose of modulation is to superimpose the desired intelligence signals on a high-frequency carrier for transmission at that high frequency from one point to another.
A signal is transmitted from one point to another for a variety of purposes. The most common example is communications between geographic areas. For a communications (e.g., telephone) circuit between two points the physical equipment usually involves an enormous quantity of hardware (e.g., poles, cross-arms, insulators, and wires). When the demand on the circuit becomes large, it is necessary to add additional facilities.
One method of increasing the facility of a circuit is to use a method of modulation. For example, a voice-frequency band from 200 to 3000 cycles is required for telephone communications. If we take bands of frequencies, 0 to 3 kc, 3 to 6 kc, 6 to 9 kc, 9 to 12 kc, and 12 to 15 kc a total band of 0 to 15 kc apparently could provide five separate channels for five separate telephone circuits over one pair of wires, provided that the original band of 200 to 3000 cycles can be transferred to each of the high-frequency bands. The process of superimposing the information contained within a frequency band onto another frequency band is called modulation. The process of decoding or converting the signal back to its original form is called demodulation or detection.
The energy medium by which the signal is to be transferred is called the carrier. The signal is often termed the modulating frequency. If we consider a single-frequency carrier, .omega..sub.o, we may write EQU e.sub.o =E.sub.m cos (.omega..sub.o t+.theta.)
When the amplitude of the carrier E.sub.m is varied in accordance with the signal information, we have amplitude modulation (AM). When the frequency of the carrier .omega..sub.o is varied in accordance with the signal, we have frequency modulation (FM). When the phase angle .theta. is varied in accordance with the signal, we have phase modulation.
Frequency modulation is superior to amplitude modulation for reducing the static and noise present in home reception of the standard AM broadcasts. Since most natural and manmade electrical noise is in the form of amplitude-modulated signals, a method of keeping the amplitude E.sub.m constant while incorporating the signal into variations of the carrier frequency .omega..sub.o accomplishes the desired noise reduction.
The frequency .omega..sub.o is modified by the signal amplitude and signal frequency. The deviation frequency, .omega..sub.F from .omega..sub.o contains the information on the amplitude or volume of the signal. The frequency of the signal, .omega..sub.M, is the rate of change of the output frequency.
These two concepts are correlated by the index of modulation, m.sub.f, as ##EQU1##
The FM output wave can be written as an infinite series of terms (carrier plus sidebands) containing Bessel functions which depend on M.sub.f and .omega..sub.M.
B. Charge Density Waves (CDW)
In most metals and semiconductors, Ohm's law and the frequency (.omega.)-independent conductivity .sigma. are the well-established and well-understood consequences of the band theory of solids. Deviations from Ohmic behavior and frequency dependent response is observed only at large electrical fields E or at relatively high (optical) frequencies, where the energy provided by the dc or ac fields is comparable with the single particle energies.
In contrast to this situation, charge transport is usually field and frequency-dependent at moderate fields and frequencies in materials where the electron structure, and consequently the conduction process, is highly anisotropic.
Low dimensional conductors or linear chain conductors are materials which have a chain structure: the solid is built of chains of atoms or molecules with strongly overlapping electronic wave functions along the chain direction, while the coupling or overlap of the wave functions between the chains is weak. This chain structure leads therefore to highly anisotropic electronic band structures.
Drastic deviations from single particle transport (due to extended electron states) are expected in linear chain compounds. In addition to the modification of the single particle dispersion relation, collective modes appear where a periodic modulation of the charge density occurs.
These collective modes appear in certain low-dimensional or linear chain conductors like TaS.sub.3 or NbSe.sub.3 which undergo a phase transition at low temperatures to a state where the electrons condense into a spatially periodic arrangement, known as the Charge-Density-Wave state (CDW). The temperature below which the system is in the charge density wave state varies from one material to another. For example, this temperature is 220.degree. K. for orthorhombic TaS.sub.3.
In these systems the CDW is pinned by the impurities present in the sample in the absence of a driving electric field. When a d.c. electric field above a certain threshold value is applied to the sample, the CDW is depinned and is able to slide through the sample, thereby providing a new mechanism of charge transport.
The sliding CDW is associated with the onset of a non-linear current-voltage characteristics and the appearance of an a.c. component in the current. The fundamental component of the a.c. current is found to be proportional to the excess current that is carried by the CDW. This a.c. component is known as the narrow-band-noise or the coherent current oscillations. The frequency spectrum of this a.c. component consists of well-defined peaks at the fundamental and its higher harmonics.
In the present invention, these characteristics of the sliding of the CDW are utilized in a method and system for the frequency modulation of a carrier frequency by a signal frequency.